1. Field of the Invention
The present invention relates to a communication-data relaying method and an inter-vehicle communication system for sending and receiving communication data containing various messages among vehicles.
2. Description of the Related Art
Inter-vehicle communication systems are known which send/receive communication data among traveling vehicles wirelessly connected to each other, as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 10-47975 (pages 14 to 28 and FIGS. 1 to 44). The inter-vehicle communication system disclosed in this Publication No. 10-47975 is provided with two communication modes, a near-vehicle communication mode and a far-vehicle communication mode, so that the user in a vehicle specifies an area to communicate with other vehicles traveling in the specified area. The near-vehicle communication mode, typically used for communication between vehicles traveling relatively close to each other, allows communication data to be wirelessly sent/received directly between such vehicles. On the other hand, the far-vehicle communication mode, typically used for communication between vehicles traveling relatively far away from each other, requires the user in a first vehicle to specify the area in which a second vehicle to communicate with is traveling, acquire the telephone number of the second vehicle, and dial the number for the second vehicle to send/receive communication data to/from the second vehicle.
Unfortunately, the vehicle to communicate with in the near-vehicle communication mode described above should be traveling within a communication radio-wave reachable range. In general, a first source vehicle can communicate with a second destination vehicle traveling beyond a radio-wave reachable range by using a third intermediary vehicle as a radio-wave relay. This approach, however, does not always ensure that data from the first source vehicle reaches the second destination vehicle, because the first source vehicle, the second destination vehicle, and the third intermediary vehicle are not moving in cooperation with one another. The first source vehicle could broadcast communication data so that nearby vehicles relay the data sequentially up to the second destination vehicle. This approach also poses a problem in that a large amount of communication data results until the data reaches the second destination vehicle, especially if the second destination vehicle is far away from the first source vehicle.
The far-vehicle communication mode described above overcomes these problems by allowing the first source vehicle to dial the number for the second destination vehicle traveling far away within mobile-phone coverage to connect specifically to the second destination vehicle. Although this approach ensures that a minimum required amount of data is used to send data from the first source vehicle to the second destination vehicle, it can be achieved only when a vehicle-mounted terminal database outside the vehicle (telephone central office, for example) is available; that is, it is difficult to employ the far-vehicle communication mode if a new infrastructure cannot be established. Thus, a new communication system has been demanded that overcomes the above-described disadvantages using vehicle-mounted devices only.